The present invention relates to a method and apparatus for manufacturing a magnetic recording medium, and particularly to a method for applying a magnetic liquid to a long wide carrier for manufacturing a magnetic recording medium.
Magnetic recording media of the so-called application type are generally manufactured by applying a liquid in a desired thickness to the surface of a nonmagnetic flexible band-like web moving continuously in a prescribed direction, and then drying the liquid on the web. As a result, a magnetic recording medium of the application type is quite different in manufacturing method from a magnetic recording medium of the non-application type manufactured using ion plating, sputtering, vacuum evaporation coating or the like. The majority of magnetic recording media for audio, video and data processing uses are manufactured using an application method.
Although an application method of the gravure type, an application method of the reverse roller type, an application method of the doctor blade type or the like can be used to manufacture a magnetic recording medium of the application type, an application method of the extrusion type is more often used because the latter provides a product of greater stability and lower cost. In a conventional application method of the extrusion type, as disclosed, for example, in Japanese Unexamined Published Patent Application No. 117669/87, a changeover valve is provided immediately upstream of an extruder used to apply the coating liquid so as to switch the passage for the liquid being applied to selectively return the liquid to a storage tank. That is, before the liquid is applied- to a web, the extruder is filled with the liquid and the changeover valve is set to return a prescribed quantity of the liquid to the storage tank under conditions similar to those as occur during the actual application time. Thereafter, the changeover valve is set to start or resume the supply of the liquid to the extruder to perform application of the liquid.
In this apparatus, at the time of starting application of liquid to the web, or when resuming application of liquid after temporarily stopping the same due to some trouble, the quantity of liquid applied to the web can be equalized to a prescribed value in a relatively short time so as to decrease the amount of waste product and thereby to enhance the production yield.
However, with such conventional application methods, it is difficult to obtain further improvements in the quality of the product and to greatly increase the speed of application of the liquid to thus further increase productivity.
There are various reasons why it is difficult to increase the speed of application. The primary reason is that the thickness of the web is very small and the liquid to be applied to form a ferromagnetic layer has a thixotropic property, which greatly affects the electromagnetic converting properties of the product. Further, in the case where the overall passageway for the liquid to be applied has a relatively large number of places where the liquid can stagnate, the liquid is likely to undergo an abnormal rise in viscosity or cohere due to the thixotropic property of the liquid, thus greatly changing the electromagnetic converting property of the ferromagnetic layer. In a conventional application device of the extrusion type, the liquid is more likely to cohere in the liquid reservoir of the extruder of the device opposite a liquid feed port for the reservoir as the quantity of the liquid discharged from the slit of the extruder is decreased. Cohesion of the liquid results in the formation of a large number of longitudinal streaks in the layer of the applied liquid on the web. If the quantity of liquid discharged from the slit of the extruder is increased, the number of longitudinal streaks tends to become smaller, but non-uniformities in the thickness of the layer of the applied liquid on the web tend to spread to the entire width of the liquid-applied portion of the web.
It is believed that the distribution of the flow speed of the liquid in the reservoir in the longitudinal direction of the extruder strongly affects the thixotropic property of the liquid. More specifically, the liquid in the reservoir near the liquid inlet side thereof, at which the flow speed of the liquid is relatively high, receives a shearing action so that the viscosity of the liquid is likely to drop, thereby resulting in non-uniformity in the thickness of the layer of the applied liquid on the web. The liquid in the reservoir at the liquid outlet side of the extruder, where the flow speed of the liquid drops to nearly zero, is likely to cohere and cause longitudinal streaks in the layer of the applied liquid on the web.
In view of the above, the present inventors have previously proposed an improved application method and disclosed the same in Japanese Patent Application No. 63601/88. In that method, a liquid is continuously discharged through the slit of an extruder which faces the surface of a continuously moving web so that a thin layer of the applied liquid is formed on the surface of the web. The method is characterized in that a quantity of the liquid larger than that applied to the web is supplied to the liquid reservoir of the extruder, which communicates with the slit thereof, and a portion of the liquid in the reservoir is forcibly withdrawn through an external drain port located at or near the end of the reservoir opposite the feed port therefor along the length of the extruder or the width of the web.
This method overcomes problems such as it being difficult to precisely set the quantity of liquid applied to the web and the quantity thereof withdrawn through the drain port due to factors such as the shape of the extruder, the physical properties of the liquid and the configuration of the liquid feed passage. Thus, the amount of non-uniformity in the thickness of the layer of the applied liquid on the web and the number of longitudinal streaks in the layer are reduced, and as a result the electromagnetic properties of the magnetic layer formed on the web are improved.
Enhancement of the yield of the product and improvement of the quality thereof are conventionally attained by mutually different methods, as described above. However, it is desired to achieve both an enhancement of yield and improvement in quality simultaneously. Using the conventional methods, however, enhancement of yield and improvement in quality could not be simultaneously attained by a combination of the two methods due reasons which will now be explained.
In the case where the methods disclosed in the abovementioned Japanese Patent Applications are combined with each other, a liquid extraction pump provided at the drain port of the extruder must remain stopped while the changeover valve provided immediately upstream of the extruder is operated so that the liquid supplied from the storage tank is all returned to the tank through a liquid return passage before the application of the liquid to the web is started. When the changeover valve is operated to start the supply of the liquid to the extruder, the liquid extraction pump is driven. However, the quantity of liquid withdrawn from the reservoir by the extraction pump does not stabilize until the rotational speed of the pump stabilizes. For that reason, it has not been possible to obtain an improvement in the yield of the product, such as could be attained by the method disclosed in the above-mentioned Application No. 117669/87. Instead, incomplete portions of the product are significantly increased, especially in the case where the layer of the applied liquid on the web is very thin, as has been the recent trend.